Two main features of the adaptation to physical training in man are a decrease in the resting heart rate and an increase in the stroke volume output of the heart. The resting cardiac output changes little. The noted increase in heart size which occurs with training strongly suggests that the increase in stroke volume is associated with an increase in end-diastolic ventricular volume. However, the contribution of other adaptive mechanisms to the increase in stroke volume is unclear. Certainly alterations in autonomic nervous system activity, atrial contribution to ventricular filling and the contractile state of the myocardium could play a significant heretofore undefined role in the production or maintenance of the increased stroke volume. Furthermore, it has not been determined whether the cardiac adaptation to physical training is associated with any significant alterations in the relationship of work performed by the heart to its energy utilization i.e., cardiac efficiency. Such questions are of obvious importance in view of the recent interest in physical training as a prophylactic or therapeutic measure to combat heart disease. Using chronically instrumented conscious animals the adaptation of the heart and its blood supply will be studied during a rigorous training program that will produce the two main cardiac effects observed in man. The adaptive changes can thus be studied during all phases of training. The data derived from these studies should provide important information concerning the mechanism of the increase in stroke volume of the heart and the question of a true cardiac beneficial effect of physical training as judged by the efficiency of the pump.